<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        function getSum() {
         let   n1 = prompt(`请输入银行卡余额`)
         let    n2 = prompt(`请输入当月食宿消费金额`)
         let     n3 = prompt(`请输入当月生活费消费金额`)
            let balance = n1 - n2 - n3
            return balance
        }

        let balance = getSum()
        document.write(`银行卡余额为${balance}`)
    </script>
</body>

</html>